Recently, as a countermeasure against global warning to solve the serious problem of the environment of the earth, active movements are promoted to save energy in the electric household appliances, machines and facilities, houses and buildings, and there is a keen demand for vacuum insulation materials having an excellent adiabatic effect for a long period.
A vacuum insulation material includes a core material having fine air gaps such as glass wool or silica powder, being covered with an laminate film having a gas barrier property, and the inside of the laminate film is sealed at a reduced pressure. The vacuum insulation material is capable of expressing a high adiabatic effect by keeping the inside space at a high degree of vacuum, and minimizing the amount of heat transferring through a vapor phase. In order to obtain its excellent adiabatic effect for a longer period, it is extremely important to maintain the high degree of vacuum in the vacuum insulation material.
Among methods of maintaining the degree of vacuum in the vacuum insulation material, generally, a method of sealing a gas adsorbent or a moisture adsorbent, together with a core material, inside the vacuum insulation material at a reduced pressure is known. By this method, the atmospheric gas can be removed from the residual moisture released from the fine air gaps in the core material after vacuum packaging into the vacuum insulation material, or from the steam, oxygen or other gas passing through the laminate film from the atmosphere and infiltrating into the vacuum insulation material at gradual time intervals.
However, considering the adsorbing capacity of the existing adsorbents, it seems insufficient by the use of adsorbents alone in order to present a vacuum insulation material capable of maintaining a high adiabatic effect for a long period, and it is necessary to add any means for suppressing the amount of atmospheric gas infiltrating into the vacuum insulation material.
Let's discuss the route of gas invading from the atmosphere into the vacuum insulation material. In the vacuum insulation material, usually, two rectangular laminate films are laid over, and outer peripheral parts near peripheral edges of three sides of the laminate films are heated and fused, and a three-way seal bag is formed. A core material is inserted into the formed three-way seal bag from its opening part, and the inside of the bag of the laminate films is evacuated by a vacuum packaging machine, and the opening part of the three-way seal bag is heated and fused.
The laminate film usually consists of an innermost layer, which is a sealant layer made of thermal plastic resin such as low-density polyethylene or the like. Its intermediate layer is a gas barrier layer made of a barrier material such as aluminum foil or aluminum deposition film. Its outermost layer is a protective layer playing the role of surface protection, such as nylon film or polyethylene terephthalate film. These layers are adhered together by way of an adhesive agent, and this laminated film is used as a laminate film.
In this case, the atmospheric gas passing into the inside of the vacuum insulation material from the atmosphere is divided into two components, that is, a component passing through pin holes in the aluminum foil or gaps in the deposition layer used as the gas barrier layer of the laminate film, and a component passing into the inside from a sealing part from an exposed portion of the sealant layer at the end of the laminate film peripheral edge.
Specifically, the thermal plastic resin composing the sealant layer is extremely high in the gas permeability and the moisture permeability as compared with the gas barrier layer. Hence, out of the atmospheric gas amount invading at time intervals into the inside of the vacuum insulation material, the majority is the portion passing into the inside through the sealing part from the exposed portion of the sealant layer at the end of the laminate film peripheral edge.
Therefore, to present a vacuum insulation material having an excellent adiabatic performance for a long period, it is indispensable to suppress the infiltration amount of atmospheric gas from the exposed portion of the sealant layer at the end of the laminate film peripheral edge, and its effective method has been studied.
To solve this problem, a vacuum insulation material has been reported by forming a thin-wall part by reducing the thickness in a part of the heat diffusion layer in the sealing part (see, for example, patent document 1).
FIG. 11 is a sectional view of the conventional vacuum insulation material disclosed in patent document 1. As shown in FIG. 11, vacuum insulation material 101 includes two laminate films 104 having gas barrier layers 102 and sealant layers 103, and a core material is sealed at a reduced pressure in a space formed by heating and fusing of them having sealant layers 103 opposite to each other. Vacuum insulation material 101 has a sealing part formed by mutual heating and fusing of sealant layers 103 by heating and pressing from the outside near the peripheral edge of two laminate films 103 so as to surround the whole circumference of the core material, and a part of sealant layers 103 in the sealing part of laminate films 104 is reduced in thickness in a prescribed width. This thin-wall part 105 is formed as shown in FIG. 12, in which a part of laminate films 104 as the sealing part is particularly heated and pressed intensively, by using sealing jig 106 consisting of an upper pattern and a lower pattern respectively having a heater at the protruding part and the inner part of an isosceles trapezoid.
In the conventional configuration, the penetration resistance of the gas invading from the end face of the laminate film peripheral edge is increased by thin-wall part 105, and the invasion of gas into inside is suppressed, and thereby it is believed that the excellent adiabatic performance may be maintained for a long period.
In patent document 1, nothing specific is mentioned is about the shape of laminate film 104 in thin-wall part 105. By using sealing jig 106 consisting of the upper pattern and the lower pattern respectively having the heater at the protruding part and the inner part of the isosceles trapezoid, a part of laminate film 104 as the sealing part is heated and pressed particularly intensively, a part of sealant layer 103 is reduced in thickness in a prescribed width, and thin-wall part 105 is formed. As a result, in thin-wall part 105, corners 107 are formed as shown in FIG. 11 and FIG. 12, and when manufacturing and handling vacuum insulation material 101, an external force is concentrated at corners 107, and cracks are formed in laminate film 104, especially in gas barrier layer 102, and the sealing part may be broken by the cracks. From such cracks or the broken position of the sealing part, in the course of a long time, invasion of atmospheric gas components into the inside of vacuum insulation material 101 may be promoted.
Herein, corners 107 are positions of angular shape (positions of a large degree of curvature) formed along with change in thickness of sealant layer 103, occurring in the boundary of thin-wall part 105 or its vicinity, when the sealing part is seen at a section cut along a plane perpendicular to the peripheral edge of laminate film 104.
Besides, since the protruding part of sealing jig 106 is an isosceles trapezoid, as compared with the portion pressed to the flat part at the leading end of the protruding part in laminate film 104, the portion opposite to the slope of the protruding part in laminate film 104 is less likely to be heated. Still more, since the majority of the portion for pressing laminate film 104 is a flat part at the leading end of the protruding part, the resin composing sealant layer 103 in the portion pressed to the flat part at the leading end of the protruding part in laminate film 104 is hardly allowed to escape to both sides. Accordingly, it was realistically difficult to reduce the thickness of thin-wall part 105 so much as illustrated.
By using sealing jig 106 consisting of an upper pattern and a lower pattern respectively having a heater at the protruding part and the inner part of an isosceles trapezoid, a part of laminate film 104 as the sealing part is particularly heated and pressed intensively, and thin-wall part 105 reduced in thickness in a prescribed thickness in part of sealant layer 103 is formed. Accordingly, the majority of the sealing part is the portion of forming thin-wall part 105, but since thin-wall part 105 is decreased in the resin for composing sealant layer 103 along the prescribed width, the adhering force for adhering two laminate films 104 together is lowered, and they may be easily peeled off by an external force. If the laminate films positioned in thin-wall part 105 are peeled off, an atmospheric gas may be easily infiltrated into the inside of vacuum insulation material 101 from the exposed portion of the sealant layer at the end face of the peripheral edge of the laminate films.
Meanwhile, gas barrier layer 102 is generally composed of a material relatively easier to transfer heat, among layers for composing laminate film 104. In particular, when gas barrier layer 102 is composed of an aluminum foil, or other metal foil or metal deposition layer, it is close to the forming portion of thin-wall part 105 along the prescribed width, and heat is likely to be transferred. That is, thin-wall part 105 plays a role of a heat bridge, and heat is likely to be transferred from gas barrier layer 102 of laminate film 104 at one heat transfer surface of vacuum insulation material 101 to gas barrier layer 102 of laminate film 104 at other heat transfer surface, and thereby the adiabatic performance of vacuum insulation material 101 is lowered.    Patent document 1: Unexamined Japanese Utility Model Application Publication No. 62-141190